I am looking for book recommendations for free reading time at school for my 6 year old science oriented son. He gets 10-15 minute slots at school throughout the day when he finishes his work quickly and instead of wasting his time waiting for others, his teacher allows him to "free read" any book from home or do an art project. He prefers to read than do art. He has taken quite a number of fiction books (Droon series, Magic Tree House etc) and a number of non-fiction books (a whole series on electricity, plants, dinosaurs) to school so far. I am running out of book choices to send in and I would love to hear your recommendations for them. I want to send in mostly non-fiction, but a few fiction books are fine too (he loves adventure). He reads at 4th grade level, his current interests are in space/planets, robots, ocean life and geography, though he will read any book with something new in it for him.
Thanks for your help!